Field of the Invention
The present invention relates to a method for controlling an exhaust gas recirculation (EGR) system for an engine, and more particularly, to a method for controlling an EGR system that may accurately monitor and calculate complex flow of EGR gas by forming and applying a formula to form a 3-dimensional map based on an equivalent orthogonal cross-section, an amount of intake air for each cylinder, and an engine speed in an EGR system which controls an intake throttle valve and an EGR valve using one motor.
Description of Related Art
Exhaust gas of an engine contains a large amount of toxic matters, such as CO, HC, and NOx (nitrogen oxides). Particularly, when a combustion temperature of the engine increases, the amount of generation of NOx increases, so that it is necessary to lower a combustion temperature of the engine in order to reduce the amount of NOx contained in the exhaust gas.
Among the reasons for the increase of the combustion temperature of the engine, a major reason is that high temperature heat is momentarily generated according to an increase of a spread speed of flames ignited by a spark plug in a state where an air-fuel ratio of an air-fuel mixed gas inside a combustion chamber is in a rich state.
A method of lowering a combustion temperature of the engine in order to reduce the amount of NOx contained in the exhaust gas includes an exhaust gas recirculation (EGR) method of lowering a combustion temperature of an engine by decreasing a density of mixed gas without changing an inherent air-fuel ratio of the mixed gas by mixing a part of exhaust gas with fresh air and making the mixed gas flow in a combustion chamber.
The exhaust gas recirculation (EGR) method is used for improving fuel efficiency of a gasoline engine, as well as reducing the amount of NOx contained in the exhaust gas. By using the exhaust gas recirculation (EGR) method, it is possible to simultaneously decrease the amount of NOx and advance ignition timing while avoiding a knocking generation region. Accordingly, it is possible to improve output of the engine and fuel efficiency.
In order to accurately control the recirculation of the exhaust gas, the amount of EGR gas recirculated to the intake manifold needs to be accurately controlled.
Among methods of controlling the recirculation of the exhaust gas, a method determines a present driving region, after pre-setting a displacement amount of a low pressure EGR valve for each driving region of an engine and forming a map table based thereon, extracts a control value from the map table, and then controls the low pressure EGR valve based on the extracted control value.
In the method, since it is required to make an EGR gas extracted from an exhaust system (extraction point) flow in an inlet (inflow point) of a compressor, a pressure difference between the two points may be insufficient for the EGR.
As shown in FIG. 1, although the insufficient pressure difference may be complemented by throttling a front side the compressor (not shown), since the amount of EGR gas non-linearly increases due to the throttling, it is difficult to accurately control the amount of EGR gas.
In FIG. 1, reference numbers 10, 20, and 30 refer to an intake throttle valve for throttling, an EGR valve for controlling the amount of the EGR gas, and a motor for controlling opening angles of the valves 10 and 20, respectively.
According to the conventional structure in which one motor such as the motor 30 controls the intake throttle valve 10 and the EGR valve 20, the amount of EGR gas passed through the EGR valve 20 may be one-dimensionally represented through the following equation.
                    m        .            EGR        =                  A        RED            ⁢                                    2            ⁢                                                  ⁢            γ                                              (                              γ                -                1                            )                        ⁢                          RT              EGRV                                          ⁢              P        EGRV            ⁢              Ψ        EGR                        Ψ      EGR        =          {                                                                                                                                                      (                                                                              P                            COMP                                                                                P                            EGRV                                                                          )                                                                    2                        γ                                                              -                                                                  (                                                                              P                            COMP                                                                                P                            EGRV                                                                          )                                                                                              γ                          +                          1                                                γ                                                                              ⁢                                                                                                    ,                                                                    P                    COMP                                                        P                    EGRV                                                  >                0.52                                                                                        0.2588              ,                                                                    P                    COMP                                                        P                    EGRV                                                  <                0.52                                                        
(ARED: equivalent cross-section, γ: specific heat ratio)
However, according to the equation, since the pressure difference (PCOMP/PEGRV) between the opposite sides of the EGR valve 20 is small, and parameters of the equation are associated with the two valves 10 and 20 and are associated with intake/exhaust flow in which pressure pulsation exists, control based on an equation θ=f(ARED) related to an opening angle (θ) of a generally-used valve and an equivalent cross-section (ARED) is difficult, as shown in FIG. 2.
The reason why the control based on the equation (θ=f(ARED)) is difficult is that an average pressure of the pressures (PCOMP, PEGRV) according to the amount of the air flow ({dot over (m)}AIR) is changeable, and a pulsation phase of the pressures (PCOMP, PEGRV) according to an engine speed (N) is changeable.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.